Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 3
Certainly, one can act from within on an illness and cure it. Only it is not always easy as there is much resistance in Matter, 
a resistance of inertia. An untiring persistence is necessary; at first one may fail altogether or the symptoms increase, but gradually the control of the body or of a particular illness becomes stronger. Again, to cure an occasional attack of illness by inner means is comparatively easy, to make the body immune from it in future is more difficult. A chronic malady is harder to deal with, more reluctant to disappear entirely than an occasional disturbance of the body. So long as the control of the body is imperfect, there are all these and other imperfections and difficulties in the use of the inner force.
If you can succeed by the inner action in preventing increase, even that is something; you have then by abhyāsa to strengthen the power till it becomes able to cure. Note that so long as the power is not entirely there, some aid of physical means need not be altogether rejected.